The present invention relates to epoxide compound resins which are oligomeric adducts of diols and diepoxides, to the amine resins thereof, and to compositions containing the amine resins which can be used in electrodeposition baths to produce the corresponding hardened resin coatings.
Cathodic electrodeposition of a film composed of amine resin, crosslinker, pigment and other resinous components onto an electrically conductive article is an important industrial process. It constitutes the usual manner in which automobile and truck bodies as well as appliance and other large metallic surface bodies are undercoated with paint. In addition to providing a painted surface, the resin systems employed protect the underlying metal surface from corrosion, impact damage and other detrimental exposure to environmental conditions.
In performing the electrodeposition, the conductive article forms one electrode and is immersed in a coating bath made from an aqueous dispersion of the film forming resin and other components. An electrical current is passed between the article and the counterelectrode in the electrodeposition bath. A charge on the article causes deposition of the resins and other components in the bath so as to produce the electrodeposit film. The deposited film is then baked or otherwise hardened to yield a coating of a substantially uniform thickness and protective characteristics.
Generally, protection from environmental and other adverse conditions is accomplished by designing into the coating resins such chemical characteristics as adhesiveness, flexibility, strength, hardness and inertness toward reaction with environmental elements. Each of these characteristics manifests itself in the protective properties of the hardened coating.
A number of advances in the protective properties of electrodeposit resin systems have been described in the patent literature. For example, U.S. Pat. Nos. 4,104,147; 4,148,772; 4,420,574; 4,423,166; 3,962,165; 4,071,428; 4,101,468; 4,134,816; 3,799,854; 3,824,111; 3,922,253; 3,925,180; 3,947,338; 3,947,339, the disclosures of which are incorporated herein by reference, described methods for improvement of the principal resin properties. The amine resin used in the coating disclosed in these patents can be flexibilized by extending the molecular length of the aromatic diepoxide starting material with polyols, with polyamines, polyether polyols, polyester polyols and other similar types of extension agents. The amine group functionality of these amine resins can also be altered according to these patents to develop protective properties. Additionally, the chemical structure of the resin cross-linking agent affects the protective properties according to these patents.
Generally, the electrodeposition bath will also contain pigment and pigment grind resins. These components are deposited as part of the coating and design characteristics are also important for them. Such patents as U.S. Pat. Nos. 3,936,405 and 4,035,275, the disclosures of which are incorporated herein by reference, as well as others describe beneficial characteristics for such grind resins.
It is also desirable to obtain as thick an electrodeposition coating as possible. Several studies have been reported in which the hardened coating thickness has been increased over that of the typical 16 micron to 20 micron film thickness produced by so-called "standard build" amine resin/polyisocyanate compositions. U.S. Pat. No. 4,487,674 (the disclosure of which is incorporated herein by reference), for example, discloses compositions for producing thick films which constitute combinations of surfactants and the amine resins disclosed in the foregoing U.S. patents.
It has been found, however, that the electrodeposition coatings described in the prior art do not achieve high corrosion resistivity and the adhesiveness for superior protection of today's vehicular bodies. It is believed these problems are the result of a two-fold difficulty. While strength and rigidity characteristics are desirable design features of principal resin system coatings, the incorporation of appropriate chemical groups providing these characteristics often adversely effects the flexibility and adhesiveness of the coatings. Consequently a balance is typically obtained between strength/rigidity and flexibility/adhesiveness. As a result, the coatings described in the prior art do not exhibit both high corrosion resistance and high strength characteristics.
Therefore, it is an object of the invention to design an amine resin system for use in an electrodeposition coating which is of high strength while also being highly resistant toward corrosion of the underlying metallic surface. It is further an object of the invention to design a principal amine resin that is flexible enough to provide a low melt viscosity and a low glass transition temperature so that uniform coatings are produced. Yet another object of the invention is the improvement of the coating adhesiveness toward the underlying substrate surface. Further objects of the invention will become apparent from the description of the invention presented herein below.